1. Field of the Invention
The present invention relates to semiconductor devices using Collector-top Heterojunction Bipolar Transistors (hereinafter referred to as C-top HBTs) in which an InGaAs base layer is employed and a collector layer is formed on the top of the surface of the InGaAs base layer. In particular, the invention relates to a method of manufacturing semiconductor devices which are ideal for power amplifiers for use in mobile communication tools for which high electric power conversion efficiency is required.
2. Description of the Related Art
Along with rapid growth of mobile communication equipment, study and development of power amplifiers for use in such communication equipment have lately been conducted actively. In order to enhance the electric power conversion efficiency of a power amplifier, it is necessary to enhance the electric power conversion efficiency of the HBTs employed in the power amplifier. For this purpose, it is effective to use HBTs employing a base layer of InGaAs (the mole fraction of InAs is 0.5) which exhibits excellent high-frequency characteristics (which will be referred to as InGaAs-base HBTs hereinafter) instead of HBTs employing a GaAs base layer which have heretofore been used commonly (which will be referred to as GaAs-base HBTs hereinafter).
Information about application of the InGaAs-base HBTs to amplifiers making a high power output is disclosed in 2001 International Conference on Indium Phosphide and Related Materials, Conference Proceedings (May 14-18, 2001, Nara), pp. 501-504.
Although the InGaAs-base HBTs have excellent high-frequency characteristics, the size of the InP substrate which lattice matches with InGaAs (the mole fraction of InAs is 0.5) is restricted to a maximum of four inches, and, therefore, the unit cost of these transistors is about three times as high as HBTs fabricated on a six-inch GaAs substrate. Because of the cost consideration, it has been difficult to apply the InGaAs-base HBTs as semiconductor devices for power amplifiers for use in mobile communication tools for which low price is required.
With respect to this problem, the chip size can be reduced by using a C-top HBT whose configuration is shown in FIG. 1 as will be described below. This C-top HBT is expected to provide a solution to the cost problem and realize a power amplifier that has good high-frequency characteristics and an excellent electric power conversion efficiency. In FIG. 1, reference numeral 1 denotes a semiconductor substrate, 2 denotes a sub-emitter layer, 3 denotes an emitter layer, 4 denotes a base layer, 5 denotes a spacer, 6 denotes a collector layer, 7 denotes a cap layer, 8 denotes a collector electrode, 9 denotes a base electrode, 10 denotes an emitter electrode, 11 denotes an external base layer, 12 denotes a high resistance external emitter.
For emitter-top HBTs (hereinafter referred to as E-top HBTs) configured by prior art, which have an emitter layer on the top of the surface of the base layer, heat does not dissipate well, and about 45μm pitches between the transistors must be provided in a multi-finger configuration of these HBTs for use in power amplifiers.
On the other hand, for C-top HBTs in which an emitter layer is formed at the bottom to make a ground plane, heat generated during the operation of the transistors can dissipate to under the substrate by providing the opening for heat dissipation which also makes the ground plane directly under the substrate of the transistors. Therefore, the C-top HBTs are of good heat dissipation and need not have an emitter electrode at the top of the surface of the base layer, and the pitches between the transistors in a multi-finger configuration of these HBTs can be reduced down to about 15 μm (see FIG. 2).
Accordingly, by using C-top HBTs, the chip size of monolithic microwave integrated circuits (MMICs) can be downsized to one third the size of the corresponding circuits using E-top HBTs. In consequence, high-efficiency power amplifiers can be produced even with InGaAs-base HBTs using InP substrates at almost the same production cost as with GaAs-base HBTs using GaAs substrates.
Information about the C-top HBTs is disclosed in, for example, IEEE Transactions on Electron Devices, Vol. 47, No. 12, Dec. 2000, pp. 2277-2283.
However, a significant problem of InGaAs-base HBTs was posed in relation to ion implantation which is an essential process for fabricating C-top HBTs. Ion implantation into the p-type InGaAs base layer causes a phenomenon that the InGaAs base layer changes to n-type or its resistance becomes higher. This phenomenon makes the ohmic contact of the base electrode inoperative and, consequently, made the transistor impossible to operate as a C-top HBT. Then, an attempt to implant p-type impurities such as berylium (Be) into the base layer again to turn it to p-type was proposed. However, this method of re-implantation of p-type impurities such as Be into the base layer cannot be applied for the following reason. In addition to higher cost by the increased number of processes, annealing at 800° C. or higher is required to activate the impurities and this has an adverse effect on the characteristics of the transistor with the InGaAs base layer because InGaAs changes its properties when exposed to temperature of 500° C. or higher.
Therefore, it has heretofore been difficult to manufacture InGaAs-base C-top HBTs having good high-frequency characteristics at low cost.